Endopyelotomy was performed in 30 patients with congenital primary ureteropelvic junction obstruction; 4 patients had high insertion of the ureter and 8 patients had caliceal stones. Clinical and radiologic success was achieved in 25 patients. There were five failures, all of whom subsequently had successful open pyeloplasty. The theoretical and experimental foundations of the procedure and fine points of the operative technique are presented. Endopyelotomy appears to be valuable for primary ureteropelvic junction obstruction just as it is for secondary obstruction.